Acute injuries to the spine and other extremities constitute both a major health risk as well as a major portion of the total health care costs in the U.S. Reducing the severity and costs of these injuries is therefore important. To address these issues, we propose to study injury biomechanics through development of advanced computer modeling and risk assessment-based etiological methods. The first area of study will develop computational models of spinal fusion in order to determine the relative importance of fusion mass location, fusion mass size, bone density of the fusion, and trabecular bone density within the vertebral body on the load carrying capacity of a lumbar interbody fusion. In the long term, these simulations can be developed into a diagnostic aid for evaluation of interbody fusions, allowing clinicians to quantitatively assess the success of the procedure and to set limits on physical activity in order to prevent re-injury of the spine, particularly for those individuals having active lifestyles. The second area of study will develop a damage-specific contrast agent, with greater x-ray attenuation than bone, for micro-computed tomography (micro-CT) of microdamage. The role of microdamage in osteoporotic fractures is not well understood, in part due to our limited capabilities for measuring microdamage non-destructive. Non-destructive techniques would enable measurement of the spatial density of microdamage accumulation with respect to local variations in mechanical loading, bone mineral density, bone architecture, whole bone geometry, or fracture sites. Consequently, basic scientific understanding of the mechanisms underlying microdamage accumulation, and the concomitant effects on fracture susceptibility, would be significantly advanced. More importantly, the development of non-destructive techniques for detecting microdamage in bone could eventually translate into new in vivo and clinical diagnostic techniques for fracture susceptibility, thus reducing the potential for injury in the elderly. The third area of study proposes developing a nonlinear hybrid cellular automata (HCA) approach for designing automotive structural topologies that are tailored for energy absorbing capability. Two application areas will be investigated: 1. The design of a knee bolster configuration for maximum efficiency; and 2. The design of a sport utility front bumper system for pedestrian safety.